The invention concerns a method for the determination of the concentration of a substance in a liquid, especially for blood sugar determination, in which the investigated fluid is applied to the test field of a test strip measured by way of electrical current, and the resulting oxidation current in the test field is captured and evaluated.
In blood sugar measurements carried out by patients themselves, a patient applies a drop of blood onto the test field of the test strip, which is then electronically measured with a suitable electronic system capturing the strength of the oxidation current which is evoked by the application of the blood onto the test field.
In the measuring device the measured current strength is integrated over a given time and the integral value is then converted into a concentration of the looked for substance. A correct value in this case is therefore only achieved if the applied amount of blood is sufficient to uniformly wet the test field. If to the contrary the test field is only partially wetted, this leads to a falsification of the measurement, since the measured current strength on the test strip is proportional to the applied amount of blood. Therefore, such a test strip is usually specified for a minimum amount of blood, which has to be applied to the test field.
By the time this fault resulting from an insufficient dosing becomes known in the case of a customary measurement, a subsequent dosing is no longer possible because in the wetted portion of the test field the chemical reaction has already so far progressed that even with a further application of blood a correct current value is no longer achievable. Moreover, in regard to the patient, usually the small puncture wound from which the blood drop has been pressed has closed to such an extent that no further blood escapes from this site. The patient must therefore repeat the measurement which for the patient is very unpleasant if one takes into mind that a patient perhaps has to carry out the same type of measurement several times a day and each time has to stick himself in the finger.
The invention has as its object the provision of a method of the aforementioned kind in which the previously described error can be recognized in good time and in the same measuring process can yet be remedied.
This object is solved in accordance with the invention in that the timewise course of the oxidation current strength is captured during a calculated time interval beginning with the application of the liquid, that a first extreme value of the current strength is compared with at least one threshold value and in that an indication is produced in dependence on the results of the comparison. The extreme value can be a minimum or a maximum value of the current strength. This depends on the kind of the electrochemical reaction in the test field. For the determination of the current strength, the current strength is preferably sensed at the ends of sample periods, which periods are short in comparison to the sensing time interval.
The magnitude of the extreme value directly after the blood application provides a representation of whether a sufficient amount of blood has been applied. If a given extreme value is not reached, this indicates the application of too little blood. Preferably, in this case the measuring device provides an indication informing the patient or the user of the device that within a given time frame a subsequent dosing onto the same test field is possible. Therefore, by means of the inventive method, the error can be recognized and remedied before the actual test reaction and measurement for the determination of the blood sugar concentration has begun.
By a comparison of the extreme value with two threshold values, for example, a given xe2x80x9cwindowxe2x80x9d can be established within which the extreme value should lie.
According to a further feature of the invention the first derivative of the sensed timewise course of the current strength is formed and its characteristics are investigated. By means of this the exact timewise position of the extreme value which appears directly after the application of the blood is determined, so that it can be used as the zero point or beginning time point for the reaction and measuring time interval, which is involved in the determination of the looked for substance. Customary systems operate, for example, in such way, that the blood application takes place with the device turned off and that the device is first turned on after the blood application. Naturally, in such a system a reproducible starting time point for a measurement is not possible, since the time point for the start of the measuring interval always depends on the time point at which the device is turned on and therefore on the user himself. So with the method described herein a substantially better reproducibility of the measuring results is provided.
The invention further concerns a device for the determination by electrical current measurement of the concentration of a substance in a liquid, especially for blood sugar determination, including a device housing with a test strip support surface for receiving an electric current measurable test strip with a test field and with electrodes connected to the test field, a measuring and evaluation circuit connected with measuring feelers for contacting electrodes of the test strip, and an indicator device controllable by the measuring and evaluation circuit, the measuring and evaluation circuit containing a program controlled computer and structured and programmed to carry out a method of the above mentioned type.
The following description explains the invention, in combination with the accompanying drawings, by way of an exemplary embodiment.